fix difference of colormap behavior across different GPUs

Author: kapadia

examples/webgl-single-image.html

@@ -50,7 +50,7 @@
     raw.setStretch('linear');
     window.raw = raw;
     // Optionally set colormap.  WebFITS defaults to grayscale mapping.
-    // raw.setColorMap('Spectral');
+    raw.setColorMap('binary');
   }
 
   function main() {

rawimage.js

@@ -714,7 +714,7 @@ rawimage = (function(){
 
   rawimage.prototype.setColorMap = function(cmap) {
     var cmaps, index, name, program, uColorIndex;
-    
+  
     cmaps = Object.keys(rawimage.colormaps);
     index = cmaps.indexOf('base64');
     cmaps.splice(index, 1);
@@ -729,7 +729,10 @@ rawimage = (function(){
       this.gl.useProgram(program);
 
       uColorIndex = this.uniforms[name].uColorIndex;
-      this.gl.uniform1f(uColorIndex, rawimage.colormaps[cmap]);
+      
+      // The color index must be offset by 0.5 since graphics cards
+      // approximate the pixel coordinate differently. 
+      this.gl.uniform1f(uColorIndex, rawimage.colormaps[cmap] - 0.5);
     };
 
     // Switch back to current program
@@ -815,6 +818,6 @@ rawimage = (function(){
 
     this.draw();
   };
-  rawimage.version = "0.4.3";
+  rawimage.version = "0.5.0";
   return rawimage;
 })();

rawimage.min.js

@@ -35,7 +35,7 @@ rawimage.prototype.loadImage = function(id, arr, width, height) {
 
 rawimage.prototype.setColorMap = function(cmap) {
   var cmaps, index, name, program, uColorIndex;
-  
+
   cmaps = Object.keys(rawimage.colormaps);
   index = cmaps.indexOf('base64');
   cmaps.splice(index, 1);
@@ -50,7 +50,10 @@ rawimage.prototype.setColorMap = function(cmap) {
     this.gl.useProgram(program);
 
     uColorIndex = this.uniforms[name].uColorIndex;
-    this.gl.uniform1f(uColorIndex, rawimage.colormaps[cmap]);
+    
+    // The color index must be offset by 0.5 since graphics cards
+    // approximate the pixel coordinate differently. 
+    this.gl.uniform1f(uColorIndex, rawimage.colormaps[cmap] - 0.5);
   };
 
   // Switch back to current program